The present invention relates to a method for delivering a melt stream of moldable plastic material under pressure through a flow passageway into a mold cavity. The plastic melt is conveyed through a distribution channel generally a hot runner or insulated runner and through a mold channel or heated nozzle communicating with the distribution channel to a mold cavity via a mold gate between the mold channel and the mold cavity.
Various means are known in the art to control the flow of resin into the mold cavities through the mold gates. U.S. Pat. No. 4,268,240 shows an air operated valve gate with a heated nozzle combined with a hot runner system for supplying resin to the mold cavity. The mold gate is opened or closed by moving a valve stem into opening or closing position with respect to the mold gate. Heated probes are also known. For example, U.S. Pat. No. 4,376,244 shows a fixed heated probe inside a channel combined with an insulated runner system for supplying a resin to the mold cavity. The gate remains in the open condition relying on ejection pressure to push out the frozen gate slug at the beginning of each injection cycle. An alternate system is shown in U.S. Pat. No. 4,517,453 which uses a fixed heated probe having external and internal heaters combined with a hot runner system. The mold gate is controlled by an internal tip heater. By turning the internal tip heater on or off, the gate can be opened or closed by alternately melting or freezing the small amount of resin in the gate orifice. An additional representative patent showing the use of a fixed heating probe is U.S. Pat. 4,795,126.
An alternate system is shown in U.S. Pat. No. 3,758,248 which uses a heated probe movable by spring pressure to close the gate and movable in the opposite direction by the injection pressure of the resin to open the gate.
Controlling the opening and closing of the mold gate in an efficient and convenient manner and in a rapid operating cycle especially involving a plurality of mold cavities, has presented problems to this art. Firstly, it is naturally highly desirable to be able to open and close the gate in an efficient and convenient manner and in a rapid operating cycle without leakage or drooling of the molten resin. Furthermore, the vestige or mark left on the molded part from the gate orifice has always represented a difficult aspect of hot runner and insulated runner molding.
Using injection pressure to open the mold gate and to compress springs has been found unreliable in multicavity molds especially because simultaneous opening of the gates is usually not achievable since spring pressures may vary from nozzle to nozzle and sometimes the probes may stick or bind in their housings, e.g., because resin has leaked into the spring cavity. Alternatively, the use of fixed probes has not been entirely satisfactory. For example, heat sensitive resins generally have narrow processing windows. Thus, when fixed probes are used in the processing of such heat sensitive resins degradation of the resin may occur in the gate area. The size of the gate orifice in combination with the proximity of the fixed heated probe thereto must be designed to both allow a quick freeze of resin in the gate area to shut off the gate and a large enough flow passage after the frozen resin is melted to allow easy filling when the gate is open. Naturally, also this must be done quickly and expeditiously in a rapid operating cycle. Too large a passage means little or even no freeze off at all, causing melted resin to drool from the mold gate. Too small a passage means restricted resin flow through the gate area causing the resin to be shear heated and often causes resin degradation during filling. Indeed, in some cases fixed heated probes cannot be used when processing heat sensitive resins.
Accordingly, it is a principal object of the present invention to provide a method and apparatus for conveniently and expeditiously delivering a melt stream of moldable plastic material through a mold gate.
It is a further object of the present invention to provide a method and apparatus as aforesaid which is readily usable on multicavity molds and which employs the use of a heated probe in the mold channel.
It is a still further object of the present invention to provide a method and apparatus as aforesaid which is readily usable with heat sensitive resins and which enables quick freeze off of the gate area and rapid opening of the gate area without an overly restricted gate passageway.
Further objects and advantages of the present invention will appear hereinbelow.